1. Field of the Invention
The present invention relates generally to the packaging of electronic components. More particularly, the present invention relates to a method of fabricating a micromachine package.
2. Description of the Related Art
Micromachine sensing elements (hereinafter micromachines) are well known. A micromachine included a miniature moveable structure, such as a bridge, cantilevered beam, suspended mass, membrane or capacitive element, which was supported over a cavity formed in a silicon wafer. Since the operation of the micromachine depended upon the moveability of the miniature moveable structure, it was critical that the package, which included the micromachine, did not contact the miniature moveable structure in any manner.
FIG. 1 is a cross-sectional view of a structure 8 during the formation of a plurality of micromachine packages in accordance with the prior art. As shown in FIG. 1, a silicon wafer 10 included a plurality of micromachine chips 12. Micromachine chips 12 included micromachine areas 14 formed in an upper surface 10U of wafer 10. Micromachine areas 14 included the miniature moveable structure of the micromachine. Micromachine chips 12 further included bond pads 16 on upper surface 10U of wafer 10. Bond pads 16 were connected to the internal circuitry of micromachine chips 12.
Micromachine chips 12 were integrally connected together in an array format. Each of micromachine chips 12 was delineated by a singulation street 20, which was located between adjacent micromachine chips 12.
A silicon lid 30 formed from a silicon wafer was positioned above wafer 10. Lid 30 included a plurality of caps 42 integrally connected to one another. Each cap 42 included a micromachine cavity 32. Each micromachine cavity 32 was positioned over a corresponding micromachine area 14. Generally, micromachine cavities 32 were wider than micromachine areas 14.
Each cap 42 further included a bond pad cavity 34. Each bond pad cavity 34 was positioned over a corresponding set of bond pads 16 on a micromachine chip 12. Generally, bond pad cavities 34 were wider than bond pads 16, and were at least as deep as bond pads 16 were tall.
FIG. 2A is a cross-sectional view of structure 8 of FIG. 1 at a further stage in fabrication in accordance with the prior art. As shown in FIG. 2A, lid 30 was attached to wafer 10. Micromachine cavities 32 were positioned above corresponding micromachine areas 14. Further, bond pad cavities 34 were positioned above corresponding sets of bond pads 16.
FIG. 2B is a cross-sectional view of structure 8 of FIG. 2A at a further stage of fabrication in accordance with the prior art. Referring to FIG. 2B, a series of shallow cuts were made to remove a portion of each cap 42 to expose bond pads 16. Micromachine chips 12 were electrically tested by connecting test probes to bond pads 16. Should testing of a micromachine chip 12 indicate that the micromachine chip 12 was defective, the micromachine chip 12 and/or corresponding cap 42 was marked. For example, micromachine chip 12A was marked as being defective. Wafer 10 was then singulated along singulation streets 20. Micromachine chips 12 which were marked as defective were discarded.
Disadvantageously, a cap 42 was attached to a micromachine chip 12 even if the micromachine chip 12 was defective. The cap 42 and defective micromachine chip 12 were discarded. However, since a cap 42 was attached to the defective micromachine chip 12, the cost associated with the defective micromachine chip 12 was increased compared to the cost associated with the defective micromachine chip 12 alone. This increased the cost of fabricating each batch of micromachine packages. This, in turn, increased the cost of fabricating each individual micromachine package which passed testing.
After singulation of wafer 10, each good micromachine chip 12 with cap 42 was further packaged. FIG. 3 is a cross-sectional view of a single micromachine package 40 in accordance with the prior art. As shown in FIG. 3, micromachine chip 12 and cap 42 were attached to a substrate 60. Bond pads 16 were electrically connected to traces 44 by bond wires 46. To prevent accumulation of static charge on cap 42, which would render micromachine chip 12 inoperable, cap 42 was electrically connected to a ground trace 48 by a bond wire 50. Ground trace 48 was grounded during use. Although effective at prevent accumulation of static charge on cap 42, grounding cap 42 by electrically connecting cap 42 to ground through bond wire 50 and ground trace 48 was relatively labor intensive and complex, which increased the cost of fabricating package 40.
A controller chip 52, which was the controller for micromachine chip 12, was also attached to substrate 60. Bond pads 62 of controller chip 52 were electrically connected to traces 44 by bond wires 46.
By integrating controller chip 52 with micromachine chip 12 into a single package 40, several advantages were realized as compared to attaching controller chip 52 and micromachine chip 12 separately as separate packages to the printed circuit mother board. One advantage was that less labor was required during component attachment to the printed circuit mother board. As a result, the cost of the electronic device employing package 40 was reduced. Another advantage was a reduction in final functional device size. However, when compared to a standard micromachine package containing only a single micromachine chip 12, i.e., without controller chip 52, package 40 was considerably larger, had reduced electrical performance and was significantly more expensive.
In accordance with the present invention, a micromachine package includes a micromachine chip having an active area, e.g., a micromachine area, in a front surface of the micromachine chip. The package further includes a controller chip having a rear surface and a front surface. An upper bond pad is on the front surface of the controller chip. A bead secures the rear surface of the controller chip to the front surface of the micromachine chip.
By mounting the controller chip directly on the micromachine chip, the size of the package is substantially reduced compared to a conventional package having a micromachine chip and controller chip in a side-by-side arrangement. More particularly, the size of the package in accordance with present invention is only slightly larger than the size of the micromachine chip.
Advantageously, a cavity above the micromachine area is formed by the bead and the controller chip. Accordingly, the cap of the prior art, which protected the micromachine area, is eliminated. Since the cap is eliminated, the prior art requirement of grounding the cap is likewise eliminated. Accordingly, the package is fabricated at a lower cost than a micromachine package of the prior art.
To further reduce the costs associated with the micromachine package, in one embodiment, a plurality of controller chips are attached to a plurality of micromachine chips while the micromachine chips are integrally connected together, e.g., while still in wafer form.
Illustratively, to attach a controller chip, a bead is applied to a rear surface of the controller chip. The controller chip further includes upper bond pads on a front surface of the controller chip. The controller chip is positioned above the micromachine area. The bead is attached to the front surface of the micromachine chip thus mounting the controller chip to the micromachine chip.
Advantageously, the controller chips are attached only to the micromachine chips which have been tested and found to be good. In this manner, waste of the controller chips is avoided and labor associated with attaching the controller chips to defective micromachine chips is saved.
In an alternative embodiment, the bead is applied to the front surface of the micromachine chip around a perimeter of the active area. The bead is contacted with the rear surface of the controller chip thus mounting the controller chip to the micromachine chip.
In either embodiment, the micromachine chip is singulated and incorporated into a lead frame type package or, alternatively, a ball grid array type package.
Also in accordance with the present invention, a micromachine package includes an upper chip mounted as a flip chip to a lower chip. In accordance with this embodiment, a plurality of traces are on the front surface of the lower chip. Upper bond pads of the upper chip are physically connected to the traces thus mounting the upper chip to the lower chip.
In one embodiment, the lower chip includes a micromachine area in the front surface of the lower chip. Alternatively, or in addition, the upper chip includes a micromachine area in the front surface of the upper chip.
A bead in combination with the upper chip and the lower chip form an enclosure, which defines a cavity. Advantageously, the micromachine area(s) in the front surface of the upper chip and/or in the front surface of the lower chip are located in the cavity and are protected from the ambient environment.
To form the package, the upper bond pads are aligned with the traces. The upper bond pads are physically connected to the traces. Illustratively, the upper bond pads are directly connected to the traces or, alternatively, are connected by flip chip bumps to the traces. A bead is formed around a periphery of the upper chip to seal and protect the micromachine area. The micromachine chip is singulated and incorporated into a lead frame type package, or, alternatively, a ball grid array type package.